Pressureless Cu@Ag sintering process with formic acid reducing agent for improved bonding of power semiconductors

Abstract

Ag and Cu are widely used sintering materials as alternatives to traditional soldering. In particular, Cu@Ag (Ag-coated Cu) has gained increasing attention as a cost-effective option. However, oxidation during the sintering of Cu@Ag and Cu DBC substrates can decrease joint strength and reliability. The addition of formic acid during the sintering process has been explored to mitigate oxidation and promote necking growth. In this study, pressureless sintering was performed using Cu@Ag to examine the effects of formic acid during the sintering process. Upon printing Cu@Ag paste on a Cu DBC substrate, an Si chip was placed on it, and sintering was performed at 300 °C in a nitrogen atmosphere. The process was tested under three conditions: introducing formic acid during pre-heating, introducing formic acid during sintering, and a control group without formic acid. The shear strength of the samples was measured post sintering. Scanning electron microscopy was used to analyze the microstructure of the sintered joints, including surface, cross-sections, and fracture surfaces after shearing. The shear strength of the sample in which formic acid was introduced during pre-heating was 1.3 times higher, measuring 21.54 MPa, compared to the condition without formic acid. This finding aligns with microstructural analysis, which showed enhanced necking formation between Cu@Ag particles. This study presents optimized pressureless sintering conditions for Cu@Ag paste, which is expected to be applicable to high-performance power module packages.